The invention relates to an exposure control circuit for a TTL (through-the-lens) automatic electronic flash, and more particularly, to an exposure control circuit for use with a camera capable of photometry of reflection from a film surface and having the capability to control the emission of light from a TTL automatic electronic flash and in which the exposure control circuit includes a diaphragm control circuit associated with the TTL automatic electronic flash for controlling a diaphragm aperture when taking pictures utilizing the automatic electronic flash.
In a conventional combination of a single lens reflex camera and an automatic electronic flash, a value of diaphragm aperture specified by the electronic flash is automatically preset in the camera in response to a charging complete signal fed from the electronic flash, and an exposure period is in fixed synchronized relationship with the operation of the electronic flash. Such technique is applicable to an arrangement in which the emission of light is controlled by the emission terminating capability of the automatic electronic flash itself.
By contrast, with a recently developed TTL automatic electronic flash in which the emission of light is controlled by an exposure control circuit of a TTL direct photometry camera, namely, a camera capable of photometry of reflection from a film surface, it is unnecessary to fix the diaphragm aperture of the camera to any specific value such as F2.8, F5.6 or the like, but any aperture may be used. The emission of light from the electronic flash occurs when the shutter is fully open so as to compensate for any insufficient illumination provided by natural light, and hence it is also unnecessary to choose an exosure period in timed relationship with the operation of the electronic flash, thus presenting a large departure from the control of light emission from the electronic flash prevailing in the prior practice.
However, when a TTL direct photometry camera capable of photometry of reflection from a film surface is used in cooperation with TTL automtic electronic flash, the electronic flash may emit light or may not emit light depending on the choice of a diaphragm aperture, except when a very bright illumination is provided to an object being photographed. Specifically, when the diaphragm aperture is reduced, the exposure supplied by natural light decreases to result in an increased length of exposure period, so that the synchro contacts are closed to cause the emission of light from the electronic flash when the shutter is fully open (synchronized with the operation of the electronic flash). Conversely, when the diaphragm aperture is increased, an increased amount of exposure supplied by natural light results in a reduced length of exposure period, failing to close the synchro contacts and thus causing the photographing operation to be performed under natural light alone.
However, since the intended purpose of having the electronic flash to cooperate with the camera is to achieve an electronic flash effect upon the photographing operation, it is desired that the electronic flash be reliably triggered during the photographing operation when the above combination is used. It is also desired that without losing the characteristics of photographing operation made with the aid of TTL automatic electronic flash, the use of the entire range of diaphragm aperture from its maximum to its minimum value be allowed.
When a photograph is taken with an electronic flash, there is a likelihood that a picture of sufficient quality is not obtained at the full-open diaphragm value by reason of the characteristics of the taking lens. Moreover, when the object being photographed is located in a near distance (for example, 2-3 m), it often occurs that a picture is over-exposed when using flashlight photography at the full-open diaphragm value. For example, the proper exposure is obtained at the guide number GN=2.8 under the condition that the full-open F number is 1.4 and the object distance is 2 m. It is difficult to properly control such a small amount of the light emission from the electronic flash at a guide number of GN=2.8.